Grinding wheel monitoring

ABSTRACT

A method of monitoring the wear of a grinding wheel comprises the steps of measuring the force exerted between the wheel and a workpiece, measured normal to the grinding face of the wheel at the point of contact between the wheel and workpiece, and generating a warning signal when the measured force exceeds a predetermined threshold value. A signal indicative of the normal grinding force is obtained by measuring the force exerted by a wheelfeed drive which in use urges the wheel into grinding engagement with the workpiece. Where the wheelfeed drive includes an electrically powered motor, the torque developed by the motor will be proportional to the normal force between the wheel and workpiece. This in turn is proportional to the electrical power drawn by the motor during operation, so an indication of the force between wheel and workpiece is obtained by measuring the power demand made by the motor on its power supply. The value of the force proportional signal obtained while grinding a workpiece is compared with a corresponding value obtained during the grinding of a preceding similar workpiece, and a warning signal is generated if a current grinding force signal value differs from a preceding grinding force signal value by more than a predetermined amount. Mean force values may be computed during each of a succession of workpiece grinds on similar components and the value for a current workpiece can be compared with the mean value for a plurality of preceding workpiece grinds. Alternatively the peak value of the normal grinding force signal value during a current grind may be compared with a predetermined value to generate a warning signal.

FIELD OF INVENTION

This invention concern methods and apparatus for monitoring the failureof grinding wheels especially Electroplated CBN grinding wheels.

BACKGROUND TO THE INVENTION

It is possible to replace the grinding material on the hub of a grindingwheel, particularly to re-electroplate a CBN wheel around the hub andthe cost of such a refurbishment of an existing hub is far less than thecost of replacing the wheel in its entirety. However if all of thegrinding material is stripped away from any part of the hub during thegrinding process, the hub cannot normally be refurbished in this way,and in particular cannot be replated with CBN material. In this event,the wheel has to be scrapped. It should therefore be of financialbenefit to an end user of grinding wheels, particularly ElectroplatedCBN wheels, to be able to predict the point in time just prior to whenthe grinding material is liable to be stripped from the hub and to allowthe machine to be stopped before the wheel is irreparably damaged.

Previously it has been thought that the most suitable method formonitoring a grinding wheel was via the increased in grind power whicharises as the wheel wears. Past tests have shown that the increase ingrind power over the life of the wheel to be about 50% but most of thisincrease is found to occur during the machining of the last half to 1%of the normal life expectancy of the wheel. Thus if the normal life of awheel is expressed in terms of the number of similar workpieces whichcan be ground by the wheel before it is worn down to the hub, and thenormal life is say 4,000 workpieces, then the 50% increase in grindpower is only found to occur during the last 20 or 30 workpieces.

This pattern is typical for a grinding wheel performing cylindricalgrinding in which the grinding face of the wheel is plain, i.e. thegrinding process is substantially uniform over the width of the wheel.

For many grinding processes, the face of the wheel is not plain, but isrequired to include at least one and sometimes two or three peripheralridges which it is found tend to wear away more quickly than theremaining surface of the wheel. This is particularly common whengrinding sidewalls with undercuts. Each of the rims of the grindingwheel have to remove considerably more metal than the central region ofthe wheel and the power increase pattern for such a wheel whenperforming this type of grinding is rather different and there is only aminimal increase in power before the grinding material is completelystripped from the wheel due to the wheel wear occurringdisproportionally over the width of the wheel.

It is an object of the present invention to provide an alternativemethod of monitoring a grinding wheel's performance which provide areliable warning of when the grinding material, particularlyElectroplate CBN material, is found to wear away, even when the wear isexcessive and uneven over the width of the wheel.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method ofmonitoring the wear of a grinding wheel comprises the steps of measuringthe force exerted between the wheel and the workpiece, measured normalto the grinding face of the wheel at the point of contact between thewheel and workpiece, and generating a warning signal when the measuredforce exceeds a predetermined threshold value.

A signal indicative of the normal grinding force may be obtained bymeasuring the force exerted by a wheelfeed drive which in use urges thewheel into grinding engagement with the workpiece.

Where the linear wheelfeed drive includes an electrically powered motor,the torque developed by which is proportional to the normal forcebetween the wheel and workpiece, this will in turn be proportional tothe electrical power drawn by the motor during operation, so that anindication of the force between the wheel and workpiece can be obtainedby measuring the power demand made by the motor on its power supply.

Where the motor is supplied with electric current from a power supplywhich maintains a substantially constant EMF, the power demand (andtherefore the normal force between wheel and workpiece) will beproportional to the current drawn by the motor from its power supply. Inthis event a force proportional signal can be obtained by measuring thecurrent flow to the motor during grinding.

In use the value of the force proportional signal obtained during agrinding process on a workpiece can be compared with a correspondingvalue obtained during the grinding process performed on a precedingsimilar workpiece, and a warning signal is generated if a currentgrinding force signal value differs from a preceding grinding forcesignal value by more than a predetermined amount.

Typically a mean value is computed for the force values measured duringeach of a succession of workpiece grinds on similar components and thevalue from the grinding of a current workpiece is compared with the meanvalue computed from a plurality of preceding workpiece grinds on similarcomponents, and the warning signal is generated if the current forcevalue differs from the mean force value by more than a predeterminedamount.

In on arrangement a timing device is reset at one point during eachgrinding process, and the force measurement is performed for a period oftime determined by the timing device following the reset point, and thevalues of these force measurement signals (or a mean of these forcemeasurement signal values) is/are compared with force measurement signalvalues from at least a preceding workpiece grind on a similar component,(or a mean of the force measurement value signals from a plurality ofpreceding workpiece grinds on similar components).

Preferably the period of time is selected to correspond to the timeduring which a part of the grinding wheel which is liable to besubjected to the greatest wear during the grinding, is in grindingengagement with the workpiece.

Where the grinding wheel includes a cylindrical surface and an annularridge for grinding an undercut in a workpiece, it will normally be theridge which is the part of the wheel surface which performs more workthan the remainder of the wheel surface and is therefore liable to thegreatest wear during grinding. When using such a wheel the timer ispreferably reset at a point during the grinding process, just in advanceof when the annular ridge is to come into contact with the workpiece.

Typically the force value signals vary in magnitude during grinding, andpreferably therefore it is the peak value of the normal grinding forcesignal value which is measured and compared with a predetermined value,and the warning signal is generated if the measured peak force valuesignal exceeds a predetermined value.

Typically the peak force signal value obtained during the grinding of atleast one of a succession of similar components is stored and isemployed as a predetermined value with which subsequent peak forcesignal values obtained from grinding each of a succession of similarcomponents, is compared.

Preferably a warning signal is only generated if the peak force signalvalue for a current grind differs from a stored peak force signal valueby more than a predetermined difference.

If generated, a warning signal may be employed to instigate a withdrawalof the wheel from grinding engagement with the workpiece.

In a preferred arrangement data logging of force is triggered X secondsafter the start of grinding each workpiece, and disabled Y seconds afterthe start of grinding, where Y is greater than X.

The invention also provides a method of monitoring grinding wheel wear,in which the instantaneous power demand of a linear motor drive whichadvances and maintains a grinding wheel in grinding contact with aworkpiece is monitored during the same part of a grinding processperformed on each of a succession of similar workpieces, and a warningsignal is generated immediately the power demand exceeds a predeterminedvalue.

The warning signal may be employed to sound an alarm to alert a machineoperator that a wheel change is required, and/or may be employed todisengage the wheel from the workpiece to prevent further wearoccurring, and/or may instigate wheel withdrawal.

Automated wheel replacement may follow by which the worn wheel isautomatically demounted from its driving spindle and withdrawn fromservice, and is replaced with a fresh wheel ready to take over thegrinding from the worn wheel.

The method of the invention is of particular use in monitoring the wearof Electroplated CBN grinding wheels, particularly such wheels which areformed with an annular groove or an annular radial protrusion, theprofile of which will grind a complementary profile in the surface of aworkpiece.

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a graph showing the normal force acting on one of two grindingwheels for an entire grind cycle;

FIG. 2 is an enlargement of the left hand end of the graph of FIG. 1;

FIG. 3 is a graph showing the nine peak forces generated by a sidewallgrind;

FIG. 4 shows the increase in normal force on the sidewall grind duringthe last 7 shafts ground using the left hand wheel of a pair bothdesigned to provide undercuts in a crankpin;

FIG. 5A and FIG. 5B respectively show part of the left hand wheel andpart of the right hand wheel of a pair of Electroplate CBN grindingwheels, each having a radial protrusion for grinding an undercut;

FIG. 6 shows a flat faced grinding wheel grinding a workpiece,

FIG. 7 is a flow diagram of a monitoring system embodying the invention,

FIG. 8 is a side view of a wheel engaging a workpiece and shows a linearmotor drive for controlling the movement of the wheel, and

FIG. 9 is a diagram showing the criteria employed by the computeralgorithm.

The graphs in FIGS. 1 to 4 were obtained from measuring the normal forceduring the grinding of a crankshaft crankpin using Electroplated CBNwheels such as shown in FIG. 5. The two wheels were used in successionwith each wheel performing half of each plunge. The undercut portion ofboth wheels performed far more work than the remainder of the wheel andtherefore in this situation it is important to monitor the grind in aperiod where only the undercut portion of the wheel is cutting. Thenormal force was monitored for the whole of each grind but data was onlyextracted during the first plunge of each wheel as this was grindinglong sidewalls.

The graph in FIG. 1 shows the left hand wheel's normal force for theentire grind cycle. The four plunges for the pins are marked due to thecycling effect of the motor force required when grinding a pin. Therapid advances and retracts, in between plunges, can be seen as thelarge peaks on the normal force plot. The section of the plot that is ofmost interest can be clearly observed at the start of the grind and iscircled in FIG. 1.

The magnified view of the circled section in FIG. 1 is shown in FIG. 2.

The same data was acquired for the right hand wheel over the full grindcycle.

In FIG. 2 it will be seen that the sidewall grind, in this case,consists of 11 force cycles followed by the large force required togrind the diameter. This data was taken for every shaft over nearly1,000 shafts at the end of which the CBN material on the left handwheel's undercut had become stripped completely to the hub. Graphs werecompiled using the values of peak force from the cycles that make up thesidewall grind. The first two force cycles were ignored as they wereoften very small or non-existent due to the variable sidewall stock. Thegraph in FIG. 3 shows the 9 peak forces generated by the sidewall grind.

It will be seen that the peak forces for the sidewall grind remainrelatively constant over the lift of the grinding wheel until just priorto wheel failure where the forces increased dramatically. The X-axis ofthe graph is the crankshaft number and in this case something in excessof 2,900 crankshafts were ground by the grinding wheels but the plot isonly from wheel 1950 through to 2,913 which was when the wheel failed.It will be seen that a huge peak in grinding force occurred just after2,900 shafts had been ground when the peak normal force which hadpreviously been of the order of 500 Newtons rose to in excess of 3,000Newtons.

The graph of FIG. 4 shows the increase in the normal force on thesidewall grind, during the last 7 shafts ground, i.e. from 2,006 to2,013 when wheel failure occurred. From FIG. 4 it will be seen that thesidewall grind forces increased dramatically over the last 5 shaftsground. If a sidewall force limit of 1,000 Newtons had been set, then awarning signal would be displayed or sounded at shaft 2,911 which wouldhave been two shafts prior to complete wheel failure. The amount ofElectroplating left on the hub at that stage is probably just sufficientto allow the wheel to be replated and yet to obtain maximum life fromthe wheel.

Since spurious force peaks can occur during grinding, it is important tomonitor the peak normal force during the same portion of each grindcycle since any response to a spurious peak occurring during anotherpart of the grind cycle will cause unwanted stoppages.

As stated previously the invention is equally applicable to flat facedgrinding wheels such as shown in FIG. 6. When grinding using a flatfaced wheel the edge region of the wheel will perform greater amounts ofwork than the central region of the wheel. The sides of the wheel willtherefore fail before the remainder of the wheel. This type ofapplication would therefore still require the windowing approachprovided by the invention.

For most grinding operations there will be a rapid advance and a rapidretract of the wheelfeed mechanism. This produces a large force peakthat needs to be eliminated from the data being monitored. Again thiswould require the windowing approach.

FIG. 6 shows by way of a flow diagram the monitoring and decision makingsteps of a wheel monitoring system embodying the invention. The systemassumes a formed CBN wheel to be grinding a formed region of acrankshaft and a linear motor wheelfeed.

The monitoring device is brought into play when the side of the wheel(the sidewall) that performs the most work in use. Therefore themonitoring device is activated once the machine starts a sidewall feedfor a journal grind.

It is to be noted that wear cannot so readily be monitored when pingrinding since in order to grind a pin the wheelhead must cycle forwardand backwards. The forwards and backwards motion masks the grindingforce data on the linear motor. At the end of a sidewall feed themonitoring is deactivated.

The signal monitored is the torque/force feedback value, direct from thelinear motor drive unit. The values used are a percentage of the maximumlinear motor current at standstill. This parameter is monitored every 30mins and compared against a preset limit value. As the signal monitoredtends to have some noise on it, then the value used to compare againstthe preset limit can be obtained by averaging the values of for examplefive total sidewall feeds.

If the preset limit is exceeded over the sidewall feeds which are to beaveraged, then the system is adapted to look for a second value thatexceeds the preset limit. At this stage the device informs the machinecontrol to immediately suspend grinding and display a message regardingimminent wheel failure.

A new wheel can then be mounted and grinding can continue.

The removed wheel can be sent for replating.

The flow diagram of FIG. 7 shows the process just described.

FIG. 8 shows a wheel 10 carried on a spindle 12 of a wheel-head 14itself carried by the primary 16 of a linear motor drive, the secondaryof which 18 is secured to the machine bed 20. Current I to the primary16 is supplied from a power supply 22 itself under the control of themachine computer 24. Grinding force between wheel 12 and workpiece 26 isproportional to the current I and since this value is available to thecomputer 22 the latter can generate an instantaneous numerical value Fproportional to I, to yield a succession of values of F. Since it isimportant for the value of F to correspond to the same point in eachgrind, the computer 24 is programmed to calculate the value of F at apredetermined stage during the grinding of each of a succession ofsimilar components. When journal grinding crank pins of crankshafts forexample, in which the wheel is employed to plunge grind between sidewalls at opposite ends of a crank pin, the value of F is calculatedduring the plunge grind since as mentioned in relation to FIG. 6, thatis when wheel wear is most likely to first become evident.

To this end the windowing is effective to prevent the value of F frombeing calculated while the flat outer face of the wheel is being used togrind the pin, after the plunge grind step, and likewise during the fastadvance and retraction of the wheel prior to and after grindingengagement.

Using experimental or wheel manufacturers data, the threshold value forF (i.e. F_(t)) is input into the computer 24 and compared with the forcevalue F and if the threshold value is exceeded a signal is generated bythe computer to instigate an audible alarm 28. If desired the samesignal may be employed to prevent the grinding of any more workpiecessuch as 26 by inhibiting the electric current to the linear motor 16, 18after the current grinding cycle has been completed and the drive 16, 18has retracted the wheelhead and disengaged the wheel from the workpiece.

The algorithm performed by the programmed computer 24 is shown in FIG.9.

In order to smooth out unexplained peaks the force value compared by thealgorithm comparison step 30 is a running average computed by summingthe latest value of F with the previous m values of F and dividing thenew value by n (where n=(m+1)).

The threshold value F_(t) is input via a data input device 32 and storedin the computer memory at 34 and compared with the running average in30.

If F_(n)/n is greater than F_(t) the comparison algorithm is satisfiedlogic produces a YES signal to generate an Alarm signal 36.

If F_(n)/n is less than or equal to F_(t) the criterion is not satisfiedand the logic produces a Grind signal 28 which enables the next grind totake place.

The windowing of the monitored value of I (and therefore the updating ofthe value of F) is controlled so as only to occur when sidewall grindingis occurring, and to this end the algorithm includes an inputcorresponding to when this is occurring at 40, which controls thecomputation of F for I in step 42 and likewise the summing of the valuesof F to produce F_(n) in 44. The division of F_(n) by n is performed in46 to provide the value of F_(n)/n which is to be compared with F_(t) in30.

1. A method of monitoring the wear of a grinding wheel in use,comprising the steps of measuring the force exerted by a wheelfeed drivewhich in use urges the wheel into grinding engagement with a workpieceso as to obtain a signal indicative of the force exerted between thewheel and workpiece normal to the grinding face of the wheel at thepoint of contact between the wheel and workpiece, and generating awarning signal when the value of the signal exceeds a predeterminedthreshold value.
 2. A method as claimed in claim 1 wherein the wheelfeeddrive includes an electrically powered motor, the torque developed bywhich is proportional to the normal force between the wheel andworkpiece, and which is in turn proportional to the electrical powerdrawn by the motor during operation, whereby an indication of the forcebetween wheel and workpiece is obtained by measuring the power demandmade by the motor on its power supply.
 3. A method as claimed in claim 2wherein the motor is supplied with electric current from a power supplywhich maintains a substantially constant EMF, so that the power demand(and therefore the normal force between wheel and workpiece) isproportional to the current drawn by the motor from its power supply. 4.A method as claimed in claim 3 wherein a force proportional signal isobtained by measuring the current flow to the motor during grinding. 5.A method as claimed in claim wherein the value of the force proportionalsignal obtained during a grinding process on a workpiece is comparedwith a corresponding value obtained during the grinding processperformed on a preceding similar workpiece, and a warning signal isgenerated if a current grinding force signal value differs from apreceding grinding force signal value by more than a predeterminedamount.
 6. A method as claimed in claim 5 wherein a mean value iscomputed for the force values measured during each of a succession ofworkpiece grinds on similar components and the value from the grindingof a current workpiece is compared with the mean value computed from aplurality of preceding workpiece grinds on similar components, and thewarning signal is generated if the current force value differs from themean force value by more than a predetermined amount.
 7. A method asclaimed in claim wherein a timing device is reset at one point duringeach grinding process, and the force measurement is performed for aperiod of time determined by the timing device following the resetpoint, and the values of these force measurement signals are comparedwith force measurement signal values from at least a preceding workpiecegrind on a similar component.
 8. A method as claimed in claim 7 whereinthe period of time is selected to correspond to the time during which apart of the grinding wheel which is liable to be subjected to thegreatest wear during the grinding, is in grinding engagement with theworkpiece.
 9. A method as claimed in claim 8 wherein the grinding wheelincludes a cylindrical surface and an annular ridge for grinding anundercut in a workpiece, and it is the ridge which is the part of thewheel surface which performs more work than the remainder of the wheelsurface and is therefore liable to the greatest wear during grinding,and the timer is reset at a point during the grinding process, just inadvance of when the annular ridge is to come into contact with theworkpiece.
 10. A method as claimed in claim wherein the force valuesignals vary in magnitude during grinding, and the peak value of thenormal grinding force signal value is measured and compared with apredetermined value, and the warning signal is generated if the measuredpeak force value signal exceeds a predetermined value.
 11. A method asclaimed in claim 10 wherein the peak force signal value during thegrinding of at least one of a succession of similar components is storedand is employed as a predetermined value with which a subsequent peakforce signal value obtained from grinding another of the succession ofsimilar components, is compared.
 12. A method as claimed in claim 10 inwhich the warning signal is only generated if the peak force signalvalue for a current grind differs from a stored peak force signal valueby more than a predetermined difference.
 13. A method as claimed inclaim 1 wherein the warning signal is employed to instigate a withdrawalof the wheel from grinding engagement with the workpiece.
 14. A methodas claimed in claim 1 wherein data logging of force is triggered Xseconds after the start of grinding each workpiece, and disabled Yseconds after the start of grinding, where Y is greater than X.
 15. Amethod of monitoring grinding wheel wear, in which the instantaneouspower demand of a linear motor drive which advances and maintains agrinding wheel in grinding contact with a workpiece is monitored duringthe same part of a grinding process performed on each of a succession ofsimilar workpieces, and a warning signal is generated immediately whenthe power demand exceeds a predetermined value.
 16. A method as claimedin claim 15 wherein the warning signal is employed either to sound analarm to alert a machine operator that a wheel change is required, or todisengage the wheel from the workpiece to prevent further wearoccurring, or to instigate wheel withdrawal.
 17. A method as claimed inclaim 15 wherein the warning signal instigates automated wheelreplacement in which the wheel is automatically withdrawn from grindingengagement, automatically demounted from its driving spindle andwithdrawn from service, and automatically replaced with a fresh wheelready to take over the grinding from the worn wheel.
 18. A method asclaimed in claim 15 wherein the monitoring is used to monitor the wearof Electroplated CBN grinding wheels.
 19. A method as claimed in claim18 wherein the wheels are formed with an annular groove or an annularradial protrusion, the profile of which will grind a complementaryprofile in the surface of a workpiece.